Various researches and proposals have been made for NOx catalysts for removing NOx by reduction from exhaust gas of internal combustion engines such as diesel engines and some types of gasoline engines and various combustion apparatuses. The de-NOx catalysts include lean NOx trap catalysts (LNT catalysts), selective catalytic reduction catalysts (SCR catalysts), and the like. The NOx occlusion reduction type catalyst is one of the lean NOx trap catalysts.
In the NOx occlusion reduction type catalyst, a catalytic metal and a NOx occlusion material which occludes NOx are supported. A basic structure of the NOx occlusion reduction type catalyst is formed by supporting a NOx occlusion material (NOx occlusion substance) having a function to occlude and release NOx and a catalytic metal which promotes the redox reaction on a catalyst support such as alumina. Examples of the NOx occlusion material include alkaline earth metals such as barium (Ba), alkali metals such as potassium (K), and the like. Meanwhile, examples of the catalytic metal include noble metals such as platinum (Pt), rhodium (Rh), and palladium (Pd).
The NOx occlusion reduction type catalyst operates as follows. Specifically, when the air-fuel ratio of the in-flow exhaust gas is in a lean (excess oxygen) state, and oxygen (O2) is present in the atmosphere, nitrogen monoxide (NO) in the exhaust gas is oxidized to nitrogen dioxide (NO2) on the metal catalyst, and the nitrogen dioxide is bound to the NOx occlusion material to forma nitrate (Ba2NO4) or the like and is occluded.
Meanwhile, when the air-fuel ratio of the exhaust gas flowing into the NOx occlusion reduction type catalyst becomes the theoretical air-fuel ratio or takes a rich (low oxygen concentration) state and the oxygen concentration in the atmosphere decreases, the NOx occlusion material such as barium is bonded to carbon monoxide (CO), which causes the decomposition of the nitrate and release of nitrogen dioxide. The released nitrogen dioxide is reduced to nitrogen (N2) with unburned hydrocarbons (HCs), carbon monoxide, or the like contained in the exhaust gas by the three-way function of the catalytic metal, so that components in the exhaust gas are released to the air in the form of harmless substances such as carbon dioxide (CO2), water (H2O), and nitrogen.
An exhaust gas purification system including a NOx occlusion reduction type catalyst performs a rich control (NOx regeneration operation) for recovering the NOx occlusion performance, when the NOx occlusion performance approaches its saturation. In the rich control, the occluded NOx is released by making the air-fuel ratio of the exhaust gas rich to lower the oxygen concentration in the in-flow exhaust gas, and the released NOx is reduced with the catalytic metal.
Here, NOx occlusion reduction type catalysts are classified into low-temperature NOx occlusion reduction type catalysts having high low-temperature activity and high-temperature NOx occlusion reduction type catalysts having high high-temperature activity, according to the characteristics of the NOx occlusion material.
In the low-temperature NOx occlusion reduction type catalyst, a NOx occlusion material mainly composed of an alkaline earth metal such as barium, which does not inhibit the activity of the catalytic metal, is used. Hence, the activity of the catalytic metal is not inhibited, and the NOx reduction performance at low temperature is excellent. However, the use of an alkaline earth metal presents a problem of decrease in NOx occlusion performance at high temperature.
On the other hand, the occlusion material used in the high-temperature NOx occlusion reduction type catalyst is an alkali metal such as potassium, which has characteristics contrary to those of alkaline earth metals such as barium. The alkali metal has high NOx occlusion performance at high temperature. However, the alkali metal inhibits the activity of a noble metal (oxidation catalyst) at low temperature, and hence presents a problem of decrease in the NOx reduction performance in a low-temperature region.
In addition, the NOx occlusion reduction type catalyst also has a problem of decrease in the NOx removal ratio due to thermal degradation (mainly, sintering). The low-temperature NOx occlusion reduction type catalyst undergoes a very slight decrease in the NOx removal ratio due to the thermal degradation in a low-temperature region (around 200° C.), but a gradual decrease in the NOx removal ratio in a high-temperature region (around 500° C.) This is because the NOx occlusion performance is effectively utilized in the low-temperature region, and hence the low-temperature NOx occlusion reduction type catalyst is not susceptible to the influence of the decrease in the occlusion efficiency due to the decrease in the activity for “NO→NO2” caused by deterioration of the noble metal.
In contrast, the high-temperature NOx occlusion reduction type catalyst undergoes a very slight decrease in the NOx removal ratio due to thermal degradation in the high-temperature region (around 500° C.), but a sharp decrease in the NOx removal ratio in the low-temperature region (around 200° C.)
With these thermal degradation characteristics taken into consideration, an exhaust gas purification system has to be designed so that the system, as a whole, can effectively conduct the NOx removal in a state with a small influence of the thermal degradation from the low-temperature region to the high-temperature region.
Considering these things, attempts to widen the temperature window of a NOx occlusion reduction type catalyst have been proposed as described in, for example, Japanese patent application Kokai publication No. Hei 10-47042, Japanese patent application Kokai publication No. 2000-167356, and Japanese patent application Kokai publication No. Hei 10-205326. Specifically, an exhaust gas purification system and an exhaust gas purification apparatus are proposed in which a high-temperature NOx occlusion reduction type catalyst on an upstream side and a low-temperature NOx occlusion reduction type catalyst on a downstream side are arranged in an exhaust passage. In addition, an exhaust gas purification catalyst apparatus for an internal combustion engine is proposed in which multiple catalysts having different NOx active temperature ranges in a lean atmosphere are arranged in series in close contact with each other, and a catalyst with a higher NOx active temperature range has a relatively larger catalyst volume and is arranged on an more upstream side.
Moreover, as described in, for example, Japanese patent application Kokai publication No. 2006-150258, the following NOx purification system is proposed to provide a NOx purification system having a wide NOx active temperature window. Specifically, in the NOx purification system, a high temperature-type NOx occlusion reduction type catalyst located on an upstream side and having a NOx occlusion material containing an alkali metal supported therein and a low temperature-type NOx occlusion reduction type catalyst located on a downstream side and having a NOx occlusion material containing an alkaline earth metal supported therein are arranged in series, and the mole ratio of platinum to rhodium of the NOx occlusion material supported on the high temperature-type NOx occlusion reduction type catalyst is set in a range from 2:1 to 1:2, both inclusive.
In addition, the NOx occlusion reduction type catalyst has the following problem. Specifically, the NOx occlusion material occludes SOx (sulfur oxides), as well as NOx. Hence, the ability to occlude NOx decreases with the increase in the amount of SOx occluded, and the NOx removal performance decreases. Since SOx is bonded to the NOx occlusion material with a greater force than NOx, desulfurization is not easy. For the desulfurization, the exhaust gas around the catalyst has to be at a high temperature and in a rich atmosphere. It is difficult to achieve the conditions of the atmosphere under operation conditions of a diesel engine, which is operated on a lean fuel in an ordinary state.
In addition, for a desulfurization control of a NOx occlusion reduction type catalyst provided alone, hydrocarbons (HCs) are supplied into the exhaust gas, and the amount of exhaust gas and the amount of the hydrocarbons supplied are controlled so that the air-fuel ratio of the exhaust gas can be stoichiometric. In contrast, in a case of the exhaust gas purification system provided with both high-temperature and low-temperature NOx occlusion reduction type catalysts, the hydrocarbons in the exhaust gas are combusted with the upstream high-temperature NOx occlusion reduction type catalyst to consume oxygen in the exhaust gas. Hence, the entire downstream low-temperature NOx occlusion reduction type catalyst is placed under a rich atmosphere which enables the desulfurization, and the desulfurization is promoted.
However, oxygen still remains in the exhaust gas flowing into the upstream high-temperature NOx occlusion reduction type catalyst. Hence, it is difficult to desulfurize the high-temperature NOx occlusion reduction type catalyst, especially at a front portion thereof, and this presents a problem of decrease in the NOx removal performance.
In addition, in a NOx occlusion reduction type catalyst, hydrocarbons or carbon monoxide is supplied as a reducing agent during the rich reduction. This presents a problem in that HC slip may occur in which part of the hydrocarbons and carbon monoxide passes through the NOx occlusion reduction type catalyst and is released to the air.